This is a nationalization of PCT/SE00/00787, filed Apr. 26, 2000 and published in English.
The present invention relates to a temperature-compensated rod resonator, a filter including such a rod resonator, and a bimetallic plate for use in such a rod resonator. More particularly, the invention concerns a rod resonator comprising:
a housing having electrically conducting walls, including side walls, a bottom wall and a top wall,
at least one electrically conductive resonator rod extending from said bottom wall towards said top wall, an upper end portion of said rod being located at a predetermined distance from said top wall, so as to define a resonance frequency,
a temperature-compensating plate located adjacent to said top wall and being adapted to change its geometrical configuration in response to temperature variations, and
coupling means for transferring electromagnetic energy to and from the resonator.
Such rod resonators are especially suitable as structural parts of filters in radio devices.
There are resonators and filters of many different kinds, e.g., cavity resonators, coaxial resonators with a central rod (for example of the kind specified above), and dielectric filters. In all these kinds of resonators attempts have been made to compensate for dimensional changes caused by temperature variations so as to keep the resonance frequency substantially constant.
A classical method is to combine various materials, having different coefficients of thermal expansion, in various portions of the resonator. Another way is to make use of bimetallic elements to achieve the desired temperature-compensation.
In a cavity resonator disclosed in U.S. Pat. No. 3,414,847 (Johnson), one of the walls defining a box-like cavity, or at least a part of such a wall, is formed by a bimetallic disc which is movable in its entirety in relation to the other walls of the cavity, primarily to enable tuning of the resonator. The disc is mounted on an axially movable plug or shaft, whereby the resonator can be tuned to a desired resonance frequency. The bimetallic disc will change its geometrical shape when the temperature varies, and the structure aims at compensating the temperature-induced dimensional changes by such a change of the shape of the disc. However, since the resonant frequency depends on the total height or length of the cavity, and the distance between the disc and the opposite wall of the cavity is relatively large, the compensating effect will vary with the particular position of the disc obtained when tuning the resonator. Therefore, it is difficult to achieve an exact temperature-compensation. Moreover, the overall dimensions of a cavity resonator of this kind are relatively large, at least in the frequency range of about 1-2 GHz.
A similar device is described in SU-836-711 (Savshinskii), where the compensating element is an elastic, cupola-shaped plate, which is peripherally secured in a metallic holder having a different coefficient of thermal expansion than that of the plate. The flexure of the plate, which is temperature-dependent, will determine the effective length of the cavity. However, the same difficulties appear as in the previous example of prior art.
Similarly,U.S. Pat. No. 3,740,677 (Motorola) discloses a cavity resonator, where a plunger on a shaft is displaceable by means of two bimetallic washers mounted on the shaft. The respective peripheral edges of the washers are secured to opposite sides of the plunger, whereby the plunger will be displaced in its entirety when the washers change their shape in response to temperature variations.
Furthermore, a dielectric resonator with a temperature-compensating bimetallic plate is disclosed in JP-3-22602. Here, the plate is mounted on a tuning screw in opposite relation to a dielectric resonator body having substantially the same diameter as the plate. Of course, in such a dielectric resonator, the major part of the electromagnetic energy is confined within the dielectric or ceramic body. Therefore, the effect of the change of the geometrical configuration of the plate is marginal. Moreover, with a relatively large tuning range, it will be virtually impossible to achieve the desired temperature-compensation so as to maintain the resonance frequency at a substatially constant value.
Another example of prior art resonators with a temperature-compensating plate is the coaxial resonator disclosed in U.S. Pat. No. 5,304,968 (LK-Products OY), which is of the kind defined in the first paragraph above. The centre part of the plate is spaced at a distance from the top wall of the resonator, and the plate has two opposite edge parts attached to the top wall. The coefficients of thermal expansion are different for the top wall and the plate. Therefore, the plate will change its configuration when the temperature varies, whereby the capacitance between the top wall and the free end of the resonator rod will be changed. However, because of the small area of the free end of the rod, it is difficult to achieve a well-defined capacitance and a precise temperature-compensation.
Against this background, a main object of the present invention is to achieve an improved temperature-compensation of a resonator of the kind defined in the first paragraph so as to keep the resonance frequency at a substantially constant value in spite of inevitable variations in temperature.
A further object is to enable the use of materials which are less temperature stable and to select suitable materials without the requirement of mixing materials having different coefficients of thermal expansion.
A still further object is to permit tuning of the resonant frequency independently of the measures required for temperature-compensation.
Yet another object of the invention is to provide a resonator having small dimensions and which is relatively easy to manufacture.
These objects are achieved for a resonator according to the invention, which has the following features:
The temperature-compensating plate is a bimetallic plate having a larger diameter than the resonator rod. The central portion of the bimetallic plate is secured to the upper end of the resonator rod, whereby the bimetallic plate, in conjunction with the adjacent top wall, defines a capacitance, which has a dominating influence on the resonance frequency while providing a reduction of the geometrical length of the rod compared to a rod without such a plate. Moreover, the peripheral portion of the bimetallic plate is permitted to be freely deflected in response to temperature variations, whereby the capacitance between the bimetallic plate and the top wall is changed so as to counteract temperature-induced dimensional changes of the housing and the resonator rod.
Tests have shown that it is possible to achieve a very stable resonance frequency with a rod resonator having such a structure. Because of the relatively large effective area of the bimetallic plate, the top capacitance (between the plate and the top wall) can be maintained at a high value while keeping a certain minimum distance therebetween, whereby the tolerances of the structural elements (the top wall and the plate) can be held at reasonable levels which facilitate the manufacturing of the resonator.
Also, the power handling capability can be increased because of the relatively large gap between the upper end of the rod and the top wall. So, the risk of a corona breakdown will be lowered.
Basically, the bimetallic plate, at least the central portion thereof, will be stationary because its central portion is fixedly secured to the top end portion of the fixed resonator rod. Even if tuning is carried out, for example by means of a tuning element located at the adjacent top wall, the bimetallic plate and the adjacent top wall are held stationary in relation to each other. Thus, in the region where the temperature compensation is performed, i.e. at the peripheral portion of the bimetallic plate, there will be no change as a consequence of the tuning process. Therefore, the temperature compensation will be substantially uneffected by the tuning.
It has turned out that the manufacture of a rod resonator according to the invention is relatively easy and inexpensive. The housing can be made of aluminium in a moulding process, and the materials for other parts of the resonator can be selected at will without considering the various coefficients of thermal expansion.
Moreover, thanks to the relatively short geometrical length of the resonator rod, the overall dimensions of the resonator, and any filter containing one or more such resonators, will be small. Of course, this is a great advantage in many practical applications, such as radio devices, for example in base stations for mobile telephone systems and the like.
From a practical point of view, it may also be advantageous to use plastic materials, coated with an electrically conductive material, for the housing and possibly also the resonator rod. Of course, the rod may be made of a different material than the housing as long as the surface portion thereof is electrically conducting.
As indicated above, it is important that the bimetallic plate is securely fastened to the top end portion of the resonator rod. This can be accomplished in a practical manner by making the bimetallic plate in the form of a ring member with a hole corresponding substantially to the cross-sectional shape of the resonator rod (at the upper end portion thereofxe2x80x94in principle, the resonator rod may have a cross-section which is different at various longitudinal sections thereof). A preferred way of securing the plate is to use a rivet connection. These and other further features will appear from the appended claims.